This patent application relates generally to electrical cables used to transmit signals between electronic devices, such as servers and routers.
Cables are often terminated at their ends with electrical connectors that mate with corresponding connectors on the electronic devices, enabling quick interconnection of the electronic devices.
A cable provides signal paths with high signal integrity, particularly for high frequency signals, such as those above 40 Gbps using an NRZ protocol. Each cable has one or more signal conductors, which is surrounded by a dielectric material, which in turn is surrounded by a conductive layer. A protective jacket, often made of plastic, may surround these components. Additionally the jacket or other portions of the cable may include fibers or other structures for mechanical support.
The components of the cable that predominately impact signal propagation, i.e., the signal conductor, the dielectric and conductive layer, are generally uniform over the length of the cable. Non-uniformities on a signal path, such as might be created by changes in shape or material of the components, give rise to changes in impedance or promote mode conversion, which reduce signal integrity, as these effects are manifested as insertion loss, crosstalk or other undesirable effects.
The signal conductor, dielectric and conductive layer are flexible, giving rise to a desirable property of cables. The flexibility enables uniform cable properties to be maintained even if the cable is routed with many bends, promoting signal transmission with high integrity.
One type of cable, referred to as a “twinax cable,” is constructed to support transmission of a differential signal and has a balanced pair of signal wires, is embedded in a dielectric, and encircled by a conductive layer. In addition to uniform dimensions of the signal wires over the length of the cable, the spacing of the wires relative to each other and to the conductive layer is maintained over the length of the cable because those components are positioned by the dielectric. Such a cable may be formed by extruding the dielectric around the signal wires.
The conductive layer is usually formed using foil, such as aluminized Mylar, or wire braid wrapped around the surface of the dielectric. The conductive layer influences the characteristic impedance in the cable and provides shielding that reduces crosstalk between signal conductors in twinax cables that may be routed together as a cable bundle. The conductive layer also forms the cable ground reference.
A twinax cable can also have a drain wire. Unlike a signal wire, which is generally coated with a dielectric to prevent electrical contact with other conductors in the cable, the drain wire may be uncoated so that it contacts the conductive layer at multiple points over the length of the cable. At an end of the cable, where the cable is to be terminated to a connector or other terminating structure, the protective jacket, dielectric and the foil may be removed, leaving portions of the signal wires and the drain wire exposed at the end of the cable. These wires may be attached to a terminating structure, such as a connector. The signal wires may be attached to conductive elements serving as mating contacts in the connector structure. The drain wire may be attached to a ground conductor in the terminating structure. In this way, any ground return path may be continued from the cable to the terminating structure.